However, if sound production is from one side of a person’s midline, the sound will reach the closer ear before the other ear and will be of a louder intensity due to the individual’s head acting as an “acoustic shadow” to dampen the noise received by the far ear. If the individual hears a sound from a location directly in the midline, the sound will reach both ears simultaneously. This crossing establishes both ipsilateral as well as contralateral input, with the majority of the fibers taking a contralateral pathway from each ear, which assists in localization and interpretation of sound quality. During this travel, information decussates or crosses to the contralateral side of the brainstem. The signal then travels to the superior olivary nucleus in the pons, up through the lateral lemniscus pathway, then to the inferior colliculus of the midbrain, and onto the medial geniculate nucleus of the thalamus, and finally synapsing in the primary auditory cortex. The auditory nerve then transmits the signal to the cochlear nucleus located between the pons and the medulla in the brainstem. Vibrations then travel to the cochlea and are sensed by inner and outer hair cells of the organ of Corti, which function to transmit the mechanical energy present in vibrational sound waves into the electrical energy transmitted along the auditory nerve. This vibration of the tympanic membrane is translated to movement and vibration of three ossicles present in the middle ear, the malleus, incus, and stapes, to further transmit vibrations to the oval window of the inner ear. As sound waves travel through the air and are collected by the pinna of the ear, they are transmitted down the external auditory canal where they will produce vibrations of the tympanic membrane. The path sound takes from the external environment to the primary auditory area and associated auditory areas, is complicated by many synapses, decussations, and inputs to brainstem nuclei bilaterally and both cerebral hemispheres. Pathway of Sound from Peripheral to Central Auditory Structures: The left Heschl gyrus in the majority of individuals is significantly longer when compared to the right gyrus, suggesting a correlation between left-hemisphere language dominance and associated differences in anatomical structure. It runs towards the center of the brain in a medial-posterior fashion. Heschl gyrus cannot be visualized from a lateral view of the cerebral cortex since it is located deep to the superficial temporal lobe structure and is within the lateral sulcus. This cortex, along with associated auditory areas, is clustered surrounding the posterior aspect of the Sylvian fissure or lateral sulcus of the cerebral cortex, which separates the temporal lobe inferiorly from the parietal and frontal lobe superiorly. The primary auditory area is housed within Heschl gyrus, a region that is positioned posteriorly in the superior temporal lobe within the supratemporal plane. The primary auditory areas are regions of the cerebral cortex located bilaterally in the temporal lobes. Structure of the Primary Auditory Cortex: The primary auditory area acts as the principal location that receives sounds from peripheral auditory structures and is integral to begin the process of complex sound interpretation as well as the conscious perception of noise. From there, through bottom-up and top-down signaling pathways, the information can be relayed to other areas of the central nervous system, cerebral cortex, and lower brainstem regions to make meaning of the information as well as integrate auditory and other sensory stimuli. For a sound to be perceived by the individual, it needs to travel to and be processed by higher-order regions in the cerebral cortex, specifically the primary auditory area. The complexity of auditory processing is apparent by the system's ability to localize, analyze, and interpret a sound that then extrapolates into useful information that the individual can respond to with simultaneous integration with other sensory stimuli. The auditory system functions to collect sound waves from the environment, transform mechanical vibrations from those sound waves into electrical nerve signals, which can be relayed to various areas of the central nervous system, and process sound into meaningful content.
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